The ability to accurately measure quantities of cryogenic fluids in a low gravity environment is a critical requirement for future space exploration missions. In low gravity, the position of the liquid in the container may be markedly different than it is in a one-g gravity field environment. Instead of settling in the bottom of the container, the fluid may become a mixture of gas bubbles interspersed within the liquid, which may be located randomly throughout the container. As a result, the familiar gauging methods used on earth are generally not applicable in space.
Many low-g quantity gauges have been investigated in concept or by laboratory testing. These systems have been based on the use of a variety of physical principles, such as radio frequency microwaves, gas bubble resonant frequency, liquid heat capacity, optical absorbency, ultrasonics, acoustics, gamma ray densitometry, and flow meters for monitoring liquids leaving and entering a tank (mass balancing). All these techniques, however, have proved to have significant limitations in gauging accuracy, complexity, or weight.
Future space mission concepts are severely limited by the inability to determine the amount of a fluid (especially a cryogen) in a tank without some form of stratification (gravity, thermal, etc.). The nature of the fluid in a low-gravity or zero-gravity environment makes metering concepts difficult. The physics governing the flow of liquids under these conditions is dominated by surface tension and viscosity forces.
Various mass gauging schemes have been proposed and/or tried. Some require complex modifications or inside surface polishing of tanks. Others rely on complex pumping devices to apply a pressure change with a piston or pump, changing the pressure inside the vessel. The volume may then be derived from ideal gas (or similar) equations of state. This requires the belief that thermodynamics only applies when it is to the benefit of the measurement. For any fluid (especially a cryogenic one), as the pressure changes so do the physical conditions in the fluid, i.e., a pressure drop would drive the transition of more gas from the liquid.
Current compression or pump mass gauging schemes are not desirable because they suggest that about ⅓ of the tank volume be displaced in order to obtain the needed accuracy using pressure transducers.
The principles of interferometry are well known. Interferometers operate by measuring the difference in the optical phase of two electromagnetic waves. A source of radiation, commonly a coherent laser source, is split and sent along separate paths. When the beams are recombined, any resulting difference in optical phase between the paths will cause the formation of fringes with a separation equal to some half-integer multiple of the wavelength.
The observed interference pattern is a composition of bright and dark bands or fringes formed by constructive and destructive interference of light between the two different optical paths. Any shift in this pattern directly relates to a change in the phase between the two paths.
It is desirable to have a mass gauging system that will work in a range of gravity and acceleration conditions.
It is desirable to have a mass gauging system that is highly accurate, functions independently of liquid orientation, and which is only minimal intrusive (e.g., does not require that a large tank volume be displaced).
It is further desirable to have a smaller piston or other compression means resulting in reduced size and weight.